Electroacoustic translating device



7 July 13, 1937. P. w. WlbLANS 2,086,834

ELECTROACOUST IC TRANSLAT ING DEVI CE Filed Nov. 7, 1954 2 Sheets-Sheet l July 13, 1937. w, w s 2,086,834

ELECTROACOUST IC TRANSLATING DEVI CE Filed Nov; 7, 1954 2 Sheets-Sheet 2 Patented July 13, 1937 UNITED STATES PATENT GFFICE Engla Application November 7, 1934, Serial No. 751,942 In Great Britain November 8, 1933 7 Claims.

It is well known that microphones behave as obstacles to the sound waves which they themselves are intended to convert into electrical oscillations, and thus give rise to distortion. At-

tempts have been made to counteract this distortion by the employment of equalizing electrical networks, but such attempts have not proved successful since the distortion diifers with the direction from which the sound reaches the microphone. A reduction of the distortion can be obtained by a reduction in the size of the microphone, but there is a practical limit beyond which the size of the microphone cannot be reduced without an undue sacrifice of robustness and sensitivity.

According to the present invention I remove the microphone proper to a distance from the point at which the sound is to be picked up and provide a sound inlet pipe of small diameter and relatively great length open at one end to the point at which sound is to be picked up and at the other end to the chamber in which the diaphragm of the microphone is mounted. Thus the only distortions of the sound waves at the point where the sound is to be picked up are those due to the end of the small diameter pipe.

The arrangement thus far described would give rise to standing .waves in the sound inlet pipe,

with consequent distortion. I therefore provide a sound absorbing chamber open to the diaphragm chamber, into which all the sound waves pass and from which they cannot return. In the case of an electrodynamic microphone the sound absorbing chamber is connected to the diaphragm chamber on the opposite side of the diaphragm to that on which the sound inlet pipe is connected. In the case of an electrostatic microphone the sound inlet pipe and the sound absorbing chamber are connected to the diaphragm chamber on the same sideof the diaphragm.

The sound conduit formed by the inlet pipe, diaphragm chamber and sound absorbing chamber should be as nearly as possible free from discontinuities and should simulate a semi-infinite conduit, into which sound can pass from one end only and from which no part of it can return. The opening from the diaphragm chamber into the sound absorbing chamber should therefore have the same area as the opening from the inlet pipe into the diaphragm chamber, and the sound absorbing chamber should be the acoustic equivalent of a tube of infinite length. Conveniently the sound absorbing chamber has the form of a 55 pipe and constitutes a smooth continuation of the inlet pipe, the only discontinuity in the sound conduit' being that due to the presence of the diaphragm chamber. The diaphragm chamber is so designed that the discontinuity which it introduces extends over as small a distance as 5 possible.

The sound inlet pipe should be long enough for the microphone proper to be remote from the point at which sound is picked up, but should not be so long as to cause undue attenuation of the 10 sound passing through it. The open end of the pipe should be at least suficiently small to avoid discrimination between sounds reaching it from different directions (that is, for high quality transmission, it should not exceed about one inch 15 in diameter), but should be sufficiently large to give the microphone the required sensitivity. I have found that a convenient length for the sound inlet pipe is three feet and a convenient diameter for its open outer end inch. The best diameter 20 for the end of the inlet pipe that is open to the diaphragm chamber will be governed by the construction of the latter; in the case of an electrodynamic. microphone it is advisable to employ a diaphragm of small area in order that the flux 5 density across the gap may be kept high, and as a consequence it is convenient to make the diameter of the input pipe near the diaphragm inch. Instead of employing an inlet pipe having a uniform diameter of 1 inch, I may employ 30 a pipe logarithmically tapered from inch at its open end to inch at the end open to the diaphragm chamber and in this way obtain an increase in response of about 6 decibels. The H pipe is conveniently made from brass and should have a smooth bore free from all irregularities.

I have found that a convenient form of sound absorbing chamber for use in conjunction with the inlet pipe above described is a pipe some 20 feet long and inch in diameter into which is 40 drawn one ply or strand of a four-ply knitting wool. Such a pipe ofiers sufficient acoustic resistance to dissipate completely the sound waves which pass into it but does not give rise to reflection at any sound frequency. The first part, say 0 two feet, of this pipe should be quite smooth and free from all irregularities; conveniently it is made of brass. The remainder of the pipe may be of lead.

One particular embodiment of the invention is illustrated in the accompanying drawings, in which Figure 1 shows a front elevation of the microphone; Figure 2 shows a plan view thereof With a part broken away; Figure 3 shows on an enlarged scale a section through the casing containing the diaphragm chamber; Figure 4 shows an elevation of the rear portion of the said casing; Figure 5 shows the device used for positioning the diaphragm.

Referring to these drawings, the casing i con taining the diaphragm chamber is mounted on the extremities of two tapered magnetic pole pieces 2 which project radially inwards from an annular permanent .magnet 3. A sound inlet pipe 4 three feet in length is open at one end to-the place at which sound is to be picked up and at the other to the diaphragm chamber At its inlet end this pipe is inch in internal diameter; it tapers logarithmically until at the end open to the diaphragm chamber it is inch in internal diameter. Also open to the diaphragm chamber is a sound absorbing chamber formed by a pipe 5 which has an internal diameter of inch and a length of 20 feet. The pipe 5 passes through a hollow drum 6 and curves gently until it reaches the periphery of this drum, the remainder of the pipe being coiled around the drum. The pipe 5 has drawn through it one ply or strand of a four ply knitting wool, this strand extending from end to end of the pipe. The first two feet of the pipe are of brass while the remainder is of lead, the two parts being connected by a junction member II.

The casing l containing the diaphragm chamb'er comprises a brass base block 1 formed on one side with a nipple 8 to which is connected the end of the inlet pipe 4 and on the other side with a projecting portion 9, which forms the bottom of the diaphragm chamber. Fixed in the block I are two pairs of steady pins i0. block l2 of slightly greater height than the projection 9 is located onthe right hand pins 10 and secured to the base block I by screws l3. A brass plate i4 provided with a projecting tag I5 is let into the upper face of an ebonite side block l6 and secured by rivets l1 and the composite block so formed, which is of the same height as the right hand side block i2, is located on the left hand pins l0 and secured to the base block 1 by screws Hi. The diaphragm l9, consisting of a strip of aluminium foil some one fifty thousandth of an inch in thickness, rests on the brass plate l4 and the right hand side block l2. The right hand end of the strip is held down by a brass side block 20 located on the right hand pins Ill and secured by screws 21 and the left hand end of the strip is held down by an ebonite block 22 located on the left hand pins l0 and secured by screws 24. A brass top plate 25 is provided on its lower face with a projecting portion 26, which is of less height than the top side plates 20 and 22 and forms the top of the diaphragm chamber, and on its upper face with a nipple 21 onto which is connected the end of the outlet pipe 5. l

The diaphragm chamber formed by the projections 9 and 26 and the right and left hand side plates is i5 millimeters in length 5 millimeters in breath and 3 millimeters in height. The diaphragm l9 extends through the length of the chamber and is inch (that is very slightly less than 5 millimeters) in width so that it just clears-the side walls of the diaphragm chamber which are formed by the end faces of the pole pieces 2.

It will be appreciated that the right hand end of the diaphragm is electrically connected to the casing i in which the diaphragm chamber is termed, while the left hand end. of the diaphragm A brass sideis insulated therefrom by the ebonite elements [6 and 22 but is in contact with the brassplate [4. Movement of the diaphragm will thus cause an electric current to flow in any circuit connected on the one hand to the casing l and on the other to the plate I4. I Normally this circuit will include the primary of a transformer, the secondary of which is connected tothe input of an amplifier.

Owing to the very thin foil of which the diaphragm is made and the consequent difliculty of handling it, I have found it necessary to provide a special device for positioning the diaphragm. This device is shown in Figure 5. It comprises a body 28, a carriage 29 slidably mounted in the body, and a screw 30 by means of which the carriage can be slidden'in the body. The body 28 is bolted to a flange 3| provided for that purpose on the base block I and the top surface of the carriage is then at the same height as the top surfaces of the lower side blocks l2 and' i 4, IS. The strip of foil which is to form the diaphragm is laid across these surfaces, and the right hand upper side block 20 and a holding block 32 on the carriage 29 are then secured so as to hold down the ends of the strip. The position of the carriage is then adjusted by means of the screw 30 until the strip, of foil has only a very slight droop between the side blocks i 2 and i4, i6. From theoretical considerations it appears permissible to set the strip just in tension, but in practice I have found it preferable to leave it slightly slack. The left hand upper side block 22 is then secured in position, the excess length of foil torn cm and the positioning device removed.

While the device illustrated and described in the foregoing has been referred to as a microphone, it may also be employed as a telephone receiver of high fidelity, which is useful as a standard of reference or for any other purpose in which the highest possible quality of reproduction is desired. When the device is thus employed, an audio frequency electric current is passed through the strip diaphragm l9 which vibrates in accordance with the electrical oscillations. The open end of the pipe 4 (which in this case is conveniently of uniform cross-section) is inserted in the ear cavity. Faithful acoustical reproduction is obtained in spite of the existence of sound reflections at the outer end of the pipe 5, since no reflections can take place elsewhere.

What I claim and desire to secure by Letters Patent is:-

1. An electro-acoustic translating device comprising a closed diaphragm chamber, an elongated sound transmitting pipe of small diameter opening directly into said chamber, means comprising a chamber containing sound absorbing material and opening into said diaphragm chamber for preventing the establishment of standing waves in the sound transmitting pipe, and means for interconverting vibrations of said diaphragm and electrical oscillations.

2. A translating .device as claimed in claim 1, wherein the cross-section of said sound transmit-. ting pipe, at its opening into said diaphragm chamber, is substantially less than the cross-section of said chamber.

3. An electro-acoustic translating device comprising a closed diaphragm chamber, a diaphragm mounted in said chamber, a sound pipe having an unobstructed bore of small diameter and relatively great length open at one end and in direct open communication at the other end with said diaphnmdnmbenaseoondflneofmsnm ter and relatively great length oumnunicaflng with saiddiamngmchsmbesymd numbing mteriniinslid ipefordilsimm stlnflaiiyemnpietelyflmsoundnmmplss intotheslmesndmennsforinta'ennvutingvibnflonsofsaiddiomngmnnd ebctrialucillotions.

LAnelectro-momflctnnshflngdevieemmq prisinga'closedehambenadiaplnngm muntedinsa'idchmbenasound lpeluving lnboreofsnniidiameterandrehtivelygreatlengthopmatmemdandindimd open mmunicsflonattheotherendwithslid ehsmbaflaseoondpipeoitheame astheinnermdoftheflrst ipeand of relatively great length communicating with said chamber, soundabsoriingmaterialinsaidsecond pipe forsubstam finityoompieteiythesoundwaveswhichposinto the same,"andmeans for inter-converting vibran ofsaid diaphragm and electrical oscillalions.

5.Anelectm-aeoustictmrflliingdevieeeumprising]. closed slumber, a. ribbon sinned oondmtlve extending scrum saidehamberandbetweenthepoksof afieidniagneLansmnd-tnnsmitflnl; pipe having anbureof smfli diameteropentonidehambu'ononesldeofsaid disphnmmdmmnsmenntlngthestshlishmmtofstsndingnvesinssidsolmd-tnnsmittingplpqnidmeanscunpddngaseoondflpe opentonidchsmberontheoppositesideof niddhphngm,andnbsorbentmatexiaiioosely' chimedinchimiwhereinsaidfirstpipeisof radon-lb decreasing with the slnnllerendthereotopentothediqahngmchamher.

'I. An demo-acoustic transhting device oomprisingachseddilphrsgmchambenaflbbon shaped'conductive extending across saidchnmberandbetweenthepolaof afleldmagneLamndpipehavinganunobstructedboreotgmdufllycmsection,'thelengthofthepipebeingoinnordersubstontinlly greater than its largest diameter, the smlllerendoisaidpipeopentosaidchamber ononesideofslidandmeanspreventingtheent ofstandingwavesin said sound pipe; said means comprising a sec ondpipeofthemmedinmeterasthesmaiier endofsaidfirstpipeopentosaidehamberonthe oppositesldeofsaiddiaphmsm, and absorbent mteriailooseiyfliiingsaidseeondpipe.

PETERWIHJAMWIILANS. 

